Cellular radiotelephone systems use a multiplicity of radio cell sites that are connected by communication links to a central office dedicated to the mobile system called a mobile switching center (MSC). This MSC is interconnected to the local public switched telephone network (PSTN) and provides mobile subscribers access to the fixed telephone network. Mobile traffic is typically communicated between the appropriate base site and the MSC over landline links, sometimes by way of intermediate switching points. The communicated traffic is essentially the voice and/or data signals transmitted to and received from the subscribers.
Traditional cell sites comprise groups of radio transceivers to transmit and receive information to and from the subscribers. Downlink (base to mobile direction), these transceivers, under remote control of the MSC, acquire one or more traffic channels by way of the information links. These traffic channel signals are suitably processed and transmitted over a radio channel by the transceiver. In the uplink direction, information is received from a radio channel by a transceiver then processed and communicated back to the MSC.
It is well known that in conventional analog cellular systems, each transceiver (which provides a single radio channel) handles only a single traffic channel of information. For voice, the necessary downlink processing might include such processing as proper audio shaping, addition of control signals, or signal compression. In newer digital systems, such as the GSM Pan-European Digital Cellular (PEDC) system, each radio channel transceiver (also supporting a single radio channel) provides multiple traffic channels using a TDMA channel structure. In PEDC there are 8 time slots for each radio channel, each supporting one full rate traffic channel. Therefore, each transceiver extracts 8 traffic signals, wherein each signal must undergo some form of processing such as channel coding, encryption, digital speech encoding, packetization, or other suitable processing. Generally, all of the functions of the cell site transceivers are partitioned on the basis of the logical traffic channels that they serve. Uplink (mobile to base direction), the signals received from mobiles on a particular radio channel are received and appropriately decoded. This information is forwarded to the MSC via a corresponding set of communication links. However, newer cellular systems provide for capabilities that previously were not provided with earlier cellular systems. Multi-frequency cellular radiotelephone systems, such as PEDC, are using frequency hopping to improve performance of the system during interference conditions and to reduce the effects of signal fading typically caused by physical obstructions or other causes of signal impairment. Frequency hopping is provided by switching a traffic channel's information (including voice and/or data) to various transmit frequencies during a conversation. This technique is well known in spread spectrum art and is particularly useful when redundant channel coding is utilized. In essence, a traffic signal is spread over a multiplicity of frequencies, known as the hopping set. Where some of the signal may experience severe degradation or interference, the coding allows acceptable signal reconstruction using only the good information that is received.
In a system such as PEDC, multiple fixed frequency transmitters may be coupled to the same antenna by using fixed tuned cavities as understood in the art. This, however, can only be done if the transmitter frequencies feeding the tuned cavities are fixed. In these types of systems, downlink frequency hopping is achieved using multiple transmitters, each tuned to a different frequency. Fixed tuned cavities provide an efficient method of combining multiple transmitters to the same antenna to simultaneously transmit multiple frequencies without experiencing the power loss typically associated with wideband passive combiners. Typically, one transmitter is allocated to one fixed tuned cavity. Therefore, when frequency hopping is used, traffic channel information must be processed, packetized, and routed to the correct transmitters belonging to a particular frequency hopping set. Routing the proper portion of the traffic channel's information to the proper transmitters has proven troublesome.
Providing this capability at a cellular base site generally requires the addition of a baseband hopping unit (BBHU) to facilitate the frequency hopping. Baseband is herein intended to include any representation of information prior to its having been modulated onto a carrier. The BBHU may be any digital switch or router, such as an additional TDM bus switch, as understood in the art. Further detail of time division switching techniques may be found in "Digital Telephony", by John C. Bellamy, Ph.d, published by John Wiley and Sons, New York, 1982. The BBHU is hard wired between traffic channel processing units and multiple RF transceivers. This additional hardware has a direct impact on the complexity of base site layouts (size of equipment cabinets), installation, fault tolerance, cost, and overall system reliability. As base sites become increasingly complex, it becomes imperative to maintain desired functionality without substantially decreasing reliability or adding substantially to the size and cost of each base site.
FIG. 1 illustrates the use of a BBHU for facilitating downlink frequency hopping in the PEDC radiotelephone system. Five carrier frequency hopping for two independent antenna sectors (170) at a single site will be used as an example. Traffic channels for the site from the Mobile Switching Center (MSC) (145) are received over the communication links (147) and put on a TDM bus (135 and 140) using bus extender apparatus (150 and 155). A group of processing units (160 and 165) is coupled to the TDM bus and receives traffic channel information that requires packetization. This traffic information is processed and packetized by the processing units (160 and 165). The BBHU (100) must then route each packet to the appropriate transmitter (115 and 120) such that multiple packets from a given traffic channel are distributed to multiple transmitters. The distribution is such that each transmitter receives its information packet(s) at a time consistent with the radio channel's burst protocol (TDMA) specified for PEDC. As previously described, the tuned cavities (125 and 130) provide for the combining of the fixed tuned transmitters onto one antenna (132 and 134).
Current systems typically utilize multiple processing units (105 and 110) to effectuate the processing functions, which at minimum includes packetization, each of which is assigned to a specific set of traffic channels and a single transceiver (115 and 120). For example, processing unit 1(160) may be assigned to transceiver 1(161) to effectuate a pre-assigned transmit scheme for eight traffic channels when frequency hopping is not required. Frequency hopping requires that the packetized information be routed to various transmitters not necessarily dedicated to that specific processing unit performing the processing and therefore requires a BBHU.
In PEDC, each radio channel supports 8 radio TDM timeslots and therefore 8 full rate traffic channels are processed in each processing unit (160 and 165). Where 5 radio channels are used for hopping, there are 40 full rate traffic channels. For any one radio TDM time frame, each of the 40 traffic channels will have a packet transmitted over one of the 5 radio channels. Over many radio TDM time frames, each radio channel transceiver in the 5 frequency hopping set will transmit information arising from all 40 traffic channels.
For uplink baseband hopping, packets associated with a single traffic channel are received over multiple frequencies by different receivers. These multiple packets must then be routed to the appropriate processing unit through the BBHU (100) whereafter the processing unit combines and processes all of the same traffic channel's packets into a single traffic signal and communicates this processed information over the TDM bus (135 and 140) to the communication link (147) and back to the MSC (145).
For the above reasons, traditional system architecture approaches have proven inadequate to economically support certain new cellular features and capability, including frequency hopping. There exists a need to facilitate frequency hopping without the use of an additional baseband hopping unit, excess processing, or additional interconnects in order to substantially increase system reliability and reduce hardware interconnect complexity and overall system cost.